In the “first study of its kind,” researchers monitored eight participants from around the world, and found that every single stool sample tested positive for the presence of microplastic.

Five women and three men—aged 33 to 65 and living in Finland, the Netherlands, the UK, Italy, Poland, Russia, Japan, and Austria—kept a nutrition diary for one week, then provided a stool sample.

All participants consumed plastic-packed food or beverages from polyethylene-terephthalate (PET) bottles; the majority ate fish or seafood, and none followed an exclusively vegetarian diet.

When tested, their excrement contained nine out of 10 types of plastics—the most common among them polypropylene (found in reusable containers, carpets and rugs, and Rubik’s Cube stickers) and PET (used for plastic bottles, flexible food packaging, thermal insulation, and frozen dinner trays).

On average, researchers found 20 microplastic particles—ranging in size from 50 to 500 micrometers—per 10 grams of feces.

“This is the first study of its kind and confirms what we have long suspected, that plastics ultimately reach the human gut,” lead researcher Philipp Schwabl said in a statement. “Of particular concern is what this means to us, and especially patients with gastrointestinal diseases.”

Microplastics are most likely to infiltrate humans via the gastrointestinal tract; once the little buggers slip into your gut, they can accumulate or transmit toxic chemicals through the body.

“While the highest plastic concentrations in animal studies have been found in the gut, the smallest microplastic particles are capable of entering the bloodstream, lymphatic system, and may even reach the liver,” according to Schwabl. “Now that we have [the] first evidence for microplastics inside humans, we need further research to understand what this means for human health.”

Global plastic production has grown rapidly since the 1950s, currently reaching heights of 400 million tons per year.

It’s still unclear, though, just what that means for people. Scientists hope to expand their research to include more study subjects and yield more direct results.

“Due to the small number of volunteers, we are unable to establish a reliable connection between nutritional behavior and exposure to microplastics,” Schwabl, of the division of gastroenterology and hepatology at MedUni Vienna, said. “The effects of the microplastic particles found on the human organism—in particular on the digestive tract—can only be investigated in the context of a larger study.”